Conventional methods of enriching and separating target gas such as hydrogen gas out of a gas mixture include a pressure swing adsorption process (hereinafter abbreviated as PSA process). The PSA process employs two to four adsorption towers in which an adsorbent is loaded, in each of which a cycle including an adsorption step, a depressurizing step, a desorption step, a scavenging step and a pressurizing step is repetitively performed. The technique of enriching and separating target gas out of a gas mixture by the PSA process can be found, for example, in JP-A 2000-313605.
The cited document discloses a technique of repeating a cycle including steps I to IX shown in FIGS. 7a to 7i, with an apparatus including three adsorption towers A, B, C respectively containing an appropriate adsorbent, so as to enrich and separate target gas and supply off-gas discharged from the adsorption towers A, B, C to an off-gas consumption unit (reformer). Specifically, these steps are carried out as follows.
In the step I shown in FIG. 7a, the adsorption step is performed in the adsorption tower A; the scavenging step is performed in the adsorption tower B; and a first depressurizing step is performed in the adsorption tower C. Specifically, a gas mixture is introduced into the adsorption tower A, so that an unnecessary gas component is removed by the adsorbent in the tower, and a product gas (target gas that has been enriched and separated) is discharged out of the tower. In the adsorption tower C, the adsorption step (see the step IX described later) has just finished, and residual gas led out from the tower C is introduced into the adsorption tower B performing the scavenging step, to serve as scavenging gas. Accordingly, the scavenging of the adsorption tower B can be simultaneously performed with the depressurizing of the adsorption tower C.
In the step II shown in FIG. 7b, the adsorption step is performed in the adsorption tower A; a first pressurizing step (pressure equalizing step) is performed in the adsorption tower B; and a second depressurizing step (pressure equalizing step) is performed in the adsorption tower C. Specifically, the adsorbent continues to remove the unnecessary gas component in the adsorption tower A as in the step I, so as to discharge the product gas out of the tower. The adsorption tower C continues to introduce the residual gas into the adsorption tower B as in the step I, so that the adsorption tower B, upon finishing the scavenging step (step I), accumulates the gas supplied from the adsorption tower C. Accordingly, the pressurizing of the adsorption tower B is simultaneously performed with the depressurizing of the adsorption tower C, so that the pressure in the adsorption tower B and the adsorption tower C may be equalized.
In the step III shown in FIG. 7c, a adsorption step is performed in the adsorption tower A; a second pressurizing step is performed in the adsorption tower B; and a desorption step (blowdown step) is performed in the adsorption tower C. Specifically, the gas mixture is continuously introduced into the adsorption tower A as in the steps I and II, and the product gas is discharged out of the tower. At this stage, a portion of the product gas is introduced into the adsorption tower B, so that pressurization is continued in the adsorption tower B. From the adsorption tower C, the residual gas is discharged out of the tower, which causes a depressurizing effect so that the unnecessary gas component desorbed from the adsorbent is discharged out of the tower.
Through the steps IV to VI shown in FIGS. 7d to 7f, the first depressurizing step, the second depressurizing step and the desorption step are successively performed in the adsorption tower A, as in the adsorption tower C through the steps I to III. In the adsorption tower B, the adsorption step is continuously performed as in the adsorption tower A through the steps I to III. In the adsorption tower C, the scavenging step, the first pressurizing step and the second pressurizing step are successively performed, as in the adsorption tower B through the steps I to III.
Through the steps VII to IX shown in FIGS. 7g to 7i, the scavenging step, the first pressurizing step and the second pressurizing step are successively performed in the adsorption tower A, as in the adsorption tower B through the steps I to III. In the adsorption tower B, the first depressurizing step, the second depressurizing step and the desorption step are successively performed, as in the adsorption tower C through the steps I to III. In the adsorption tower C, the adsorption step is continuously performed as in the adsorption tower A through the steps I to III.
Repeating the foregoing steps I to IX in the adsorption towers A, B, C leads to removal of unnecessary gas component from the gas mixture, and to continuous extraction of the product gas containing a high concentration of the target gas.
Meanwhile, the off-gas discharged from the adsorption tower B in the step I, adsorption tower C in the step III, adsorption tower C in the step IV, adsorption tower A in the step VI, adsorption tower A in the step VII, and adsorption tower B in the step IX is supplied to the reformer as a fuel, after being temporarily stored in an off-gas reservoir tank (not shown). The off-gas reservoir tank is installed in order to store a portion of the off-gas discharged in the above steps so as to secure the continuous supply of the off-gas to the reformer without interruption in the steps II, V and VIII, in which none of the adsorption towers discharges the off-gas.
However, the off-gas reservoir tank normally has a capacity more than five times as large as that of the adsorption tower, and has hence been a great obstacle in achieving further reduction in dimensions of the target gas enrichment and separation system. Besides, according to the technique disclosed in the cited document, reducing the capacity of the off-gas reservoir tank inevitably incurs an increase in pressure fluctuation, and is hence difficult to achieve.